The present invention relates in general to diode-laser light-sources and in particular relates in general to a light source for projecting light from a linear diode-laser array into an elongated line of light.
Diode-lasers are commonly used as sources of illumination in various graphics applications such as display systems, optical printing systems and optical recording systems. In one type of prior-art imaging system, a linear (one-dimensional) array of light modulators is illuminated by an illuminator including a light source. Illumination from the array of modulators is projected onto a recording medium or the like in the form of a line of images of the modulators. By scanning the recording medium past the line of images and appropriately synchronizing the scanning with operation of the modulator array, the modulator array is used to draw a two-dimensional image on the recording medium.
A preferred light-source for the illuminator is a linear array of diode-lasers commonly referred to as a diode-laser bar. A diode-laser bar can efficiently provide relatively high illumination power, for example 60.0 Watts (W) or more, from a source having a maximum overall dimension no greater than about 10 centimeters (cm). A disadvantage, however, is that each of the diode-lasers in the bar is an individual emitter. This presents problems in selecting an appropriate optical configuration for the illuminator.
This problem is addressed in one prior-art illuminator by including a diode-laser array (or correspondingly an array of light-emitting diodes) having a number of emitters equal to the amount of modulators to be illuminated, and an optical system configured to image each emitter onto a corresponding modulator. This type of illuminator has a disadvantage that it is dependent on all emitters continuing to function. Failure of one emitter could cause a reduction in performance of the recording system, for example, a black line on an image in the direction of scanning.
An illuminator apparatus for overcoming this disadvantage is disclosed in U.S. Pat. No. 5,517,359 (Gelbart et al.). Here an optical system is used to project elongated overlapping images of each individual emitter onto a modulator array. The number of emitters need not correspond to the number of modulators, and failure of an individual emitter is described as merely reducing the overall illumination on the modulator rather than effecting the spatial distribution of illumination on the modulator.
Another illuminator apparatus for overcoming this disadvantage is described in a co-pending application Ser. No. 09/522,120, assigned to the assignee of the present invention by inventors Rekow et al. The Rekow et al. arrangement comprises a diode-laser bar including a linear array of diode-lasers (emitters). An optical system is arranged with a longitudinal axis thereof parallel to the propagation direction of light from the diode-lasers. The optical system has mutually perpendicular axes designated fast and slow axes fast and corresponding to fast and slow axes of the diode-lasers. The optical system is arranged to focus fast-axis diverging rays of the diode-lasers in a focal plane perpendicular to the longitudinal optical axis, and arranged to form the slow-axis diverging rays of the diode-lasers into a plurality of bundles of parallel rays, one thereof for each diode-laser. The bundles of parallel rays intersect in a focal plane of the optical system. The optical system thereby causes light from the diode-lasers to be formed into a line of light in the focal plane. The line of light has a width in the fast axis of the optical system and a length in the slow axis of the diode-lasers.
The line of light has a distribution of intensity of illumination similar in form to that of the far-field intensity distribution of an individual emitter. This intensity distribution is not optimally uniform, but has a somewhat Gaussian distribution of intensity modified by a central reduction of intensity. In other embodiments of the Rekow et al. apparatus, the distribution of intensity along the line of light is rendered somewhat more uniform by including one or more physical stops arranged to prevent selected portions the fast and slow-axis diverging rays from the diode-laser array from reaching the fast-axis focal plane of the optical system. The selected portions are chosen to optimize the intensity of light distribution along the length of the line of light.
While somewhat effective in improving the intensity distribution along the line of light, the distribution is not believed to be optimum and comes at the expense of wasting light from the diode-laser array that is prevented by the physical stops from reaching the focal plane of the optical system. There is a need to for a diode-laser bar powered illuminator that projects a line of light wherein the intensity of illumination in the line is substantially uniform but that does not require the inclusion of physical stops to achieve this uniformity of illumination.
The present invention is directed to an illumination apparatus for projecting a line of light. In one aspect, the inventive apparatus comprises a plurality of diode-lasers each thereof having an emitting-aperture and arranged in an elongated linear array. The diode-lasers being spaced apart by a spacing defined as the distance between the centers of adjacent diode-lasers in the array. The diode-laser array has a slow axis parallel to the length direction of the array, and a fast axis perpendicular to the slow axis. Light is emitted from an emitting aperture each of the diode-lasers as diverging rays in both the fast and slow axes in a general direction of propagation mutually perpendicular to both the fast and slow axes. The inventive optical apparatus includes an optical system including a first anamorphic lens, and a lens group including a second anamorphic lens and a spherical lens having positive optical power. The optical system has a longitudinal axis parallel to the propagation direction of light from the diode-lasers and has mutually perpendicular fast and slow axes corresponding to the fast and slow axes of the diode-laser array. The optical system further includes one or more arrays of microlenses providing two arrays of cylindrical refracting surfaces. The one or more arrays of microlenses are located between the first anamorphic lens and the lens group. The arrays of cylindrical refracting surfaces have a pitch defined as the distance between vertices of adjacent ones of the cylindrical surfaces. The pitch of the microlens array is different from the spacing between the diode-lasers. The cylindrical refracting surfaces are arranged such that each thereof receives rays from a plurality of the diode-lasers and such that rays leaving the one or more microlens arrays arrive at the lens group as rays diverging in both the fast and slow axes. The lens group is arranged to focus the fast-axis diverging rays of the diode-laser array in a focal plane perpendicular to the longitudinal optical axis, and is arranged to form the slow axis diverging rays into a plurality of converging bundles of parallel rays intersecting in the focal plane. Accordingly, light from the diode-lasers is formed into a line of light about in the focal plane. The line of light has a width in the fast axis and a length in the slow axis.
Preferably, each of the first and second anamorphic lenses of the optical system has greater optical power in the fast axis than in the slow axis, and most preferably has positive optical power in the fast axis and zero optical power in the slow axis. Preferably, each of the microlenses in the one or more microlens arrays has zero optical power in the fast axis and positive optical power in the slow axis.
In one preferred embodiment of the inventive illumination system, there is only one microlens array and each of the microlenses in the microlens array has first and second cylindrical surfaces. In another preferred embodiment of the inventive illumination system there are two microlens arrays and each of the microlenses in the microlens array has a plane first surface and cylindrical second surface.